The present invention relates to a copper base alloy which is superior in resistances to seizure, wear and corrosion, suitable for use as a material of a sliding member, particularly, sliding members which are used under severe sliding conditions such as a floating bush bearing of a turbocharger.
In general, the following materials (1) to (3) are known as materials for a floating bush bearing of a turbocharger: (1) a free cutting brass (JIS H3250), (2) a lead-bronze (JIS H5115) and (3) a low-friction high-tension brass which is disclosed in JP-B2-53-44135 and JP-B2-56-11735 of earlier patents of the same applicant.
However, the above-mentioned alloy (1) is inferior in resistances to seizure and wear when used under a boundary lubricating condition, and the alloy (2) can not provide sufficiently high resistance to corrosion when used in deteriorated lubricating oil at high temperature. The alloy (3) could not exhibit satisfactorily high resistance to seizure because of difficulty in enhancing its lead content due to the fact that this alloy has a micro-structure of which matrix is composed of a mixture of .alpha.- and .beta.-phases or .beta.-phase alone.
In recent years, there is a remarkable trend for supercharged engines, and floating bush bearings which are used in turbochargers attached to internal combustion engines are required to operate under more severe conditions such as ambient temperature of the environment, rate of supply of the lubricating oil and degradation of lubrication oil.
In general, a floating bush bearing is heated to a high temperature, e.g., 400.degree. C. or so, due to transfer of heat from a turbine, so that sulfur contained in a lubricating oil, depending on a nature of the oil and the temperature, tends to react with copper in the bearing metal so as to form copper sulfide (CuS), forming a blackened layer mainly composed of CuS on the surface of the bearing metal. This is known as the "blackening corrosion phenomenon". The blackened layer progressively grows as the bearing is used long and is exfoliated from the bearing surface to seriously impair the bearing function of the floating bush bearing.
Furthermore, the conventional bearing materials could not provide satisfactory seizure resistance under dry conditions, i.e., when the lubrication with lubricating oil is stopped at a high temperature such as 300.degree. C. or higher. To explain in more detail, a turbocharger, which has a gas turbine impeller driven by energy of exhaust gas of high temperature and pressure and a compressor driven by the turbine impeller, has to idle due to its inertia even after the engine is stopped to terminate the supply of pressurized lubricating oil. Consequently, the turbocharger is obliged to idle awhile without cooling and lubricating effects produced by the lubricating oil. As a result, heat energy which has been accumulated in the turbine housing of high temperature is transmitted to regions of lower temperatures, thus raising the temperature of the bearing portion. Thus, the bearing is required to have a high resistance to seizure in dry state at high temperature.
Hitherto, lead-bronze systems containing copper, lead and tin as main components, and free cutting brass containing copper, zinc and lead as main components have been widely used as the material of the floating bush bearings of turbochargers. However, floating bush bearings of lead-bronze system alloy undesirably promote generation of blackened layer due to reaction between sulfur in the lubricating oil and copper in the bronze under dry-up condition at high temperature of 300.degree. C. or so and allows a rapid wear of the bearing surface. On the other hand, the free cutting brass system alloys, although they exhibit superior corrosion resistance, exhibit inferior affinity for lubricating oil after termination of lubrication, thus allowing a comparatively early seizure or scuffing.